1. Field of the Invention
The present invention relates to a motor whose rotor is rotated for driving.
2. Description of the Related Art
A spindle motor, an example of a motor, generally has a housing and a hub member rotatable relative to the housing, which are disposed with a bearing means provided therebetween. A rotor magnet is mounted on the hub member, while a stator is mounted on the housing in such a manner as to face the rotor magnet. A labyrinth seal structure is also provided between the housing and the hub member. The labyrinth seal structure prevents grease or the like from the bearing means from seeping into an accommodation chamber accommodating a recording member such as a magnetic disk.
However, a spindle motor of this type, in which a labyrinth seal structure is especially provided for the above-stated purpose between the housing and the hub member, entails various problems. For example, the motor has a complicated construction, and is expensive. Another problem is that a labyrinth seal structure cannot be provided if the housing is of a certain kind of structure. In such a case, the occurrence of contamination may not be adequately prevented.
Another type of spindle motor has a magnetic fluid seal structure instead of a labyrinth seal structure. Such a magnetic fluid seal means has magnetic fluid retainer means comprising a permanent magnet and a pair of pole pieces disposed on either side of the permanent magnet. The magnetic fluid retainer means is disposed on an annular bush member formed of aluminum, which bush member is mounted on an inner peripheral surface of a hub member. An electrically conductive adhesive is coated over the bush member and the magnetic fluid retainer means, and the bush member and the magnetic fluid retainer means are electrically grounded through the adhesive (the static electricity generated by the rotation of a recording member is grounded through the hub men-her and the adhesive to a frame of the associated driving apparatus).
The provision of such an electrically conductive adhesive, however, necessitates the following: it is necessary to use two types of adhesives, that is, an adhesive for securing the magnetic fluid retainer means, and an electrically conductive adhesive for grounding. It is also necessary to perform an adhering process in two steps. In these adhering steps, it is necessary that the amount of the adhesive as well as the position at which it is to be coated be controlled by means of a plurality of kinds of jigs or machines handled by a highly skilled worker.
If a motor, such as a spindle motor, includes a radial ball bearing as a bearing means, there is a risk that a non-repeating deflection may occur. When the housing of the motor has been subjected, during manufacture, to a grounding operation while being held by a three-pawl chuck, the radius of the housing may have three cycles of undulations (i.e., three crests and three troughs) per 360.degree.. When the housing has such undulations, the undulations may be transferred to the outer ring of the radial ball bearing press-fitted on the inner periphery of the housing, thereby involving the risk that a rolling-ball race on the outer ring may also have similar undulations in the radial direction thereof.
When the rolling-ball race on the inner or outer ring has such radial undulations, the undulations may induce a non-repeating deflection when the motor is rotated. Such a deflection occurring in a spindle motor, such as a spindle motor for driving a hard disk, may cause an off-track action, which may in turn cause a reading or writing error.